1. Field of the Invention
The invention relates generally to floating offshore structures. More particularly, the invention relates to unconditionally stable buoyant semi-submersible platforms and tension leg platforms for offshore drilling and production.
2. Background of the Technology
Conventional semi-submersible offshore platforms and tension leg platforms include a hull that has sufficient buoyancy to support a work platform above the water surface, as well as rigid and/or flexible piping or risers extending from the work platform to the seafloor, where one or more drilling or well sites are located. Whether a semi-submersible or tension leg platform, the hull typically includes a plurality of horizontal pontoons that support a plurality of vertically upstanding columns, which in turn support the work platform above the surface of the water. For example, in FIG. 1A, a conventional offshore platform 10 for the drilling and/or production of hydrocarbons includes a hull 20 that supports a work platform 30 above the sea surface 11. Hull 20 is formed from a plurality of generally horizontal pontoons 21 extending between a plurality of generally vertical columns 22. In general, the size of the pontoons and the number of columns are governed by the size and weight of the work platform and associated payload to be supported. For tension leg platforms, the columns primarily function to provide buoyancy, while the tendons provide stability (e.g., resist excessive tilting/listing of the platform). For semi-submersible offshore structures, the pontoons function as the primary source of buoyancy, while the columns (and associated spacing) provide stability. For most semi-submersible and tension leg platform, each column typically includes an opening at its upper end above the sea surface. Such openings may include access trunks allowing personnel access entry into the column; hawse pipes permitting chain to be pulled into and stores in chain lockers within the columns; ventilation pipes and ducts; or combinations thereof. These openings may permit seawater to flood the column either from a wave washing over the top of the column or from seawater entering the column due to excessive vessel heel.
Wind and wave excitation forces at and below the sea surface continuously seek to move offshore structures. Translational movement of semi-submersible platforms at the sea surface is typically limited by mooring lines extending from the platform to the sea floor, and translational movement of tension leg platforms at the sea surface is typically limited by tendons that extend from the platform to the sea floor and are placed in tension. Mooring lines allow for some vertical movement of the semi-submersible structures (e.g., heave) relative to the sea floor, while tendons restrict and/or prevent vertical movement of tension leg platforms relative to the sea floor. Wind and wave excitation forces may also cause offshore structures (e.g., semi-submersible or tension leg platforms) to tilt or list to one side. For example, in FIG. 1B, offshore platform 10 is shown tilting or listing to one side due to wind and wave forces acting on platform 10. The angle through which the offshore structure tilts relative to vertical, neutral is often referred to as the “heeling” angle, and is designated as angle α in FIG. 1B. If the heeling angle is sufficiently large, the offshore structure may capsize with potentially catastrophic effects.
For semi-submersible platforms, the geometry and arrangement of the columns operate to resist excessive heeling and restore the platform back to its upright, neutral position. However, with extreme wind and/or wave excitation forces, heeling angles can be quite large. At a sufficiently large heeling angle sea water is allowed to flow directly from the sea into one or more openings in the top of the columns of the platform. The smallest heeling angle at which the opening in the upper end of one or more columns is positioned at the sea surface is often referred to as the “downflooding” angle, and is designated as angle β in FIG. 1C. When the heeling angle is equal to or greater than the downflooding angle, the uncontrolled flooding of one or more columns further exacerbates listing, and may cause the platform to capsize. For tension leg platforms, the tendons operate to resist excessive heeling and restore the platform back to its upright, neutral position. However, in some cases, one or more tendons may fail, potentially allowing the platform to tilt to the downflooding angle.
Accordingly, there remains a need in the art for offshore platforms that are unconditionally stable and able to resist capsizing. Such offshore platforms would be particularly well-received if they were unconditionally stable, regardless of the geometry and arrangement of the columns and integrity of tendons.